Exterior automotive windows are subject to many foul byproducts of travel. Emmissions, dirt, grime from other vehicles, bugs, small animals all leave a film that detracts from the overall apperance of the vehicle and is a safety consideration.
Tools & Technique
To clean exterior automotive windows use a regular commercially available window cleaner and cotton rags. Old cloth diapers are the best, but any thick cotton cloth that has been previously washed will clean without leaving lint.
The secret is to wash the window twice. The first wash will remove dust, dirt, and road grime. On the second washing use only a limited amount of spray, and polish the window until all of the streaks are gone.
There are also a number of excellent products available that will treat the windows so water will sheet off. These products need reapplying as often as monthly, depending on the amount of driving you do in the rain. Although inexpensive, the treatments take time to apply, but the added safety and convenience are worth while for many drivers.